The NEXRAD system is comprised of a series of 159 Weather Surveillance Radar–1988 Doppler (WSR-88D) sites situated throughout the United States as well as selected oversees sites. The NEXRAD system is a joint venture between the United States Department of Commerce (DOC), the United States Department of Defense, (DOD) as well as the United States Department of Transportation (DOT). The individual agencies that have control over the system are the NWS, Air Force Weather Agency (AFWA) and the FAA. [Figure 12-23]
NEXRAD radar produces two levels of products: level II and level III.
Level II Data Products
All NEXRAD level-II data products are available through the National Climatic Data Center (NCDC). Level II data consists of the three meteorological base data quantities: reflectivity, mean radial velocity, and spectrum width.
Level III Data Products
There are 41 products routinely available through the NCDC. Level III graphic products are available as digital images, color hard copy, gray scale hard copy, or acetate overlay copies. This information is then encoded and disseminated through the satellite weather system as well as other sources.
NEXRAD level III data for up to a 2,000 mile range can be displayed. It is important to realize that the radar image is not real time and can be up to 5 minutes old. At no time should the images be used as storm penetrating radar nor to navigate through a line of storms. The images display should only be used as a reference.
NEXRAD radar is mutually exclusive of Topographic (TOPO), TERRAIN and STORMSCOPE. When NEXRAD is turned on, TOPO, TERRAIN, and STORMSCOPE are turned off because the colors used to display intensities are very similar.
Lightning information is available to assist when NEXRAD is enabled. This presents a more vivid picture of the weather in the surrounding area.
In addition to utilizing the soft keys to activate the NEXRAD display, the pilot also has the option of setting the desired range. It is possible to zoom in on a specific area of the display in order to gain a more detailed picture of the radar display. [Figure 12-24]
Although NEXRAD is a compilation of stations across the country, there can be abnormalities associated with the system. Some of the abnormalities are listed below.
- Ground clutter
- Strobes and spurious radar data
- Sun strobes, when the radar antenna points directly at the sun
- Interference from buildings or mountains, which may cause shadows
- Military aircraft which deploy metallic dust and may reflect the radar signature
In addition to the abnormalities listed, the NEXRAD system does have some specific limitations.
The NEXRAD base reflectivity does not provide adequate information from which to determine cloud layers or type of precipitation with respect to hail versus rain. Therefore, a pilot may mistake rain for hail.
In addition, the base reflectivity is sampled at the minimum antenna elevation angle. With this minimum angle, an individual site cannot depict high altitude storms directly over the station. This will leave an area of null coverage if an adjacent site does not also cover the affected area.
The resolution of the displayed data will pose additional concerns when the range is decreased. The minimum resolution for NEXRAD returns is two kilometers. This means that when the display range is zoomed in to approximately ten miles, the individual square return boxes will be more prevalent. Each square will indicate the strongest display return within that two kilometer square area.
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